INDUSTRIAL d N D ENGINEERING CHEMISTRY
1140
tion assumes a lower value. I n the case of steel 0 is comparatively large, probably in the neighborhood of 80 degrees. As pointed out in the preceding paragraphs, if 0 has a low value the tread stock does not carry the load without undue distortion and hence will not give satisfactory wear on a pneumatic tire. It has also been shown that steel, where e is comparatively great, does not wear well because the forces a t the instant of contact are high enough to assume the dominating role. It is interesting to speculate as to t,he optimum values 0 may have in an ideal tread stock. I n other words, for given values of resilient energy and the concavity factor, how large may become before the impact forces offset the advantages gained by producing a stock t o resist distortion? Likewise the question may be propounded -for use in linings to resist abrasion, how small may 0 become if the resilient energy can be maintained a t a comparatively high value? The analysis of tread wear discussed in the preceding paragraphs should aid in correcting an impression frequently voiced by rubber technologists that the extraordinarily
Vol. 18, No. 11
high elongation of rubber compounds before rupture is unnecessary. If the arguments presented above are sound, high stretch is essential. We may say with confidence to the organic chemist that, for use in tread stocks a t least, compounds containing substitutes for rubber must show high resilient energy and a comparatively high rigidity. Moreover, if his stocks are to be employed as a wear-resisting material in lined chutes for sand, gravel, and other abrasives, the resilient energy should be as high as is consistent with rather low rigidity. Resistance to tear is doubtless another important factor in tread wear and in withstanding the action of abrasives. I t seems probable that this factor is intimately connected with high tensile and elongation. There is, however, some lack of unanimity among rubber technologists on this point. Acknowledgment
The author gratefully acknowledges helpful suggestions by R. E. Day and valuable aid by R. L. Moore in the preparation of this article.
Composition and Structure of Hevea Rubber By R. P. Dinsmore THEGOODYEAR TIRE& RUBBERC O , AKRON,OHIO
T
In 190j Willstatters showed cyclooctadiene was a ring of eight carbon atoms. Harriesg a t this time was able to form levulinic aldehyde and acid from ozonide, and this was taken as evidence that the structure of rubber was 1,j-dimethyl cyclooctadiene. Harries reasoned that if, as he believed, two isoprenes condensed to form the above compound, two butadienes should give Willstatter’s cyclooctadiene. This proved to be the case. A t this time Harries proposed the partial valence explanation of polymerization-two partial valences for each double bond. In 1910 Pickleslo raised objection to partial valence theory because: (1) polymerization does not reduce unsaturation and (2) polymers persist after destructive distillation, indicating strong bonds. Pickles proposed a long chain structure with free end bonds. In 1912 Barry and Weidert’l worked out a formula for vulcanized rubber, based on the amount of sulfur (2.5 per cent) required t o render rubber insoluble in benzene. The formula was (C10H16)2S2= 2500, from which x would equal 18+, In 1921 “alpha is0 rubber” (CloHl~)zwas prepared by Harries.12 Molecular weight showed z = 8. This was apparently not a true hydride, hut in 1922 rubber hydride was prepared by Pummerer and Burkard.’3 It behaved somewhat like rubber but oxidized readily. In 1922 a nonoxidizing hydro rubber was prepared by Staudinger and Fritchi.I4 A compound C~oHloowas isolated from its decomposition products, indicating high mo,lecular weight of original polymer and strong polymer bonds. This caused Staudinger to postulate primary valence linkage. C h e m i c a l Composition I n 1924 methyl and ethyl hydro rubber were made by THE RUBBERHYDROCARBON-In 1860 Greville \Villiamsl * Staudinger and Widmer.l5 obtained isoprene and couchine from rubber by distillation. In 1925-26 rubber heated to 250-270’ C. was shown by I n 1879 Bouchardotz condensed isoprene to dipentene Staudinger and Geiger16 to form a product they called “polyIn 1884-5 Wallach3 proved dipentene and couchine to be cyclo rubber,” losing all but one-fifth of its double bohds. “ I n identical. ternal cyclization” .was proposed as an explanation by StaudI n 1892 Tilden4 found t h a t dipentene decomposed to form inger. isoprene and t h a t isoprene could be polymerized to a rubberI n 1926 Fisher and Gray17 also made polycyclo rubber a t like material. temperatures 340-345” C. Harries5 in 1902 obtained two isomers of dipentene from rubber. In 1926 Staudinger and Widmer’s showed that the hydride of Gladstone and Hibbertc in 1888 analyzed rubber and showed polycyclo rubber can be formed. Staudinger and Geigerlg show the purified material corresponded very closely to ( C S H S ) ~ . that gutta-percha and balata yield .the same hydro and polycyclo They also obtained optical evidence t h a t there were three double rubber as Hevea. It may be pointed out here that all known bonds for ten carbon atoms. Their compounds were not abderivatives of the rubbers just mentioned are the same regardless solutely pure. Harries’ in 1904 obtained more accurate information regarding of the rubber. Recent work of Staudinger and Wehrli (private comrnunicaunsaturation by formin’g rubber ozonide, which he proved to be tion) shows t h a t traces of bromine or trichloroacetic acid cause C10H1606 thus indicating only two double bonds for ten carbon enormous reductions in the viscosity of a rubber cement and atoms. solutions of other large molecules. They cite this fact as evidence of a long hydrocarbon chain with end valences too weak t o * Numbers in t e x t refer to bibliography a t end of article.
HAT human minds are led into error alike by inadequate
hypotheses and by faulty ones is not a new observation. Yet the frequency with which its truth is demonstrated is sufficient excuse for calling it to mind. If the fact is well known, it is not treated with sufficient consideration. I n the study of rubber we find that progress has been hampered by hasty generalization and lack of a suitable hypothesis. Some investigators, obtaining a few facts which indicate the importance of chemical composition, jump to the conclusion that nothing else matters. Others make the same assumption regarding physical structure. Still others, intent on accumulating facts, misinterpret conditions and obtain a distorted image of the facts observed because they have no well-defined idea of the relation of their observations to the group of phenomena which constitute the whole. It is therefore believed that a better knowledge of the composition and structure of rubber mill be promoted by an orderly presentation of the facts nom known and, where the evidence is meager or contradictory, by calling attention to such discrepancies. By this means the errors in present hypotheses will be made evident and the foundation laid for a more suitable assumption.
Kovember, 1926
INDLr9TRI.4L A N D EiVGILVEERIArGCHEMISTRY
close the ring. Rubber dissolved in various solvents gives various viscosities. Staudinger attributes this t o variation in size of macromolecule. I n 1926 rubber isomers were made by action of sulfuric acid by Kirchhof.zO In 1926 various forms of cyclo rubber differing in degree of saturation were made by Staudinger and Geiger,21and Widmer.I8 In 1926 Fisher, Gray, and McColmzz showed the reaction product of phenol and rubber hydrobromide to be R(C6HaOH)p. Geiger23 has shown also that triphenyl phosphonium hydrobromide rubber behaves like a true salt, also t h a t mixtures of acids of high molecular weight can be made from partly hydrogenated rubber. In the writer's laboratories Brusont has prepared new isomers of ruhber by the use of such catalysts as SnCl4, SbClj, FeC18, Ticla, etc. Rubber forms highly colored complex products with these compounds. When the catalyst is split off with alcohol new isomers of rubber are obtained, which are snowwhite powders, totally unlike rubber physically, almost completely satiirated toward bromine, and analyzing very exactly (C5H8),. These white compounds will again form highly colored complex products when treated with the catalysts mentioned. The original complex products are easily oxidized in air, forming insoluble oxides, but they are stable in an inert atmosphere. The white isomers range from extremely soluble to insoluhle, depending on the catalyst employed. With a given catalyst, more than one isomer may be obtained from rubber depending upon experimental conditions. This work is illuminating for several reasons. For one thing, these halide catalysts will not react with substances which do not possess residual valences or with completely saturated hydrocarbons. An example of a substance easily polymerized (by sunlight) but which will not react with stannic chloride, for example, in the dark, is vinyl bromide. The fact that rubber reacts with these halides indicates residual valences and the fact that the reaction goes in a few minutes a t room temperature indicates powerful forces. Of still greater interest is the formation of two isomers from rubber. It remains t o be shown in what proportion these isomers occur and whether one can be converted into the other. The formation of similar compounds from synthetic rubber and from isoprene again shows the chemical similarity of these materials and rubber. h70 doubt balata will behave in an identical manner.
Examination of the data herein presented leads to the conclusion that the fact is well established t,hat rubber is a polymer of C6&. If we neglect the end valences, t'he evidence seems strong that the linkage holding together the C5Hgunits is not directly dependent upon the other double bonds, because polymerization does not lessen their activity; also saturation of the double bonds does not reduce the size of the polymer, a t least to anything close to isoprene, but when the double bonds are partly or wholly saturated the character of the rubber changes markedly. There is good evidence of the chemical identity of Hevea, balata, and guttapercha. Furthermore, there is reason to believe that there exists in rubber more than one polymeric magnitude and that the. size of some polymers is very great'. Evidence of the kind of polymer linkage is insufficient. and the type of polymer structure, whether ring or chain, is uncert,ain. More evidence should be obtained either t o reenforce or combat the present preferred standing of the long-chain, endvalence hypothesis. A study of the evidence bcariiig on physical structure will throw some further light on this general question.
1141
I
same as Tschirch and Schmitz, but concluded that the protein was a glycoprotein. They found protein from Para rubber more reactive than from plantation. Dekker**in 1916 accomplished a separation by heating t o 230260' C. in petroleum oil Frankz9in 1914 accomplished a separation by boiling in cymene or limonene. He reports results of hydrolysis with sulfuric acid which have not been confirmed. Belgrave30 in 192j reports a comprehensiw scheme for determining the chemical nature of rubber protein. He used on latex the recent method of Kingston and S ~ h r y v e r which , ~ ~ is hydrolysis with sulfuric acid and precipitation of dicarboxylic acids by phosphotungstic acid. The amino acids are separated by alcohol and proline is left in solution. His results show that there are a number of compounds in latex protein with no particular predominance of any one. In further work he prepared a protein-poor rubber by dilution of latex, adding alcohol, and precipitating with an elect1 olyte. The rubber contained 0.06 to 0.09 per cent nitrogen. The protein obtained from the latex serum contained 11.6 per cent nitrogen and no inorganic phosphorus could be split off. This arviies against phosphoprotein; other evidence indicates that rubber protein is not a glycoprotein. The precipitate obtained from the hydrolyzed protein treated with amyl alcohol and ether gave the following: Amide nitrogen Humin nitrogen Arginine Cystine
Per cent 7.7 0.5 22.4
Per cent I
This also shows varied composition, though the protein may not be in the original form. Beadle and Stevens32 showed that the nitrogen content of rubber affects the amount of sulfur combined in a 3-hour vulcanization period Nitrogen, per cent Combined sulfur, per cent
0 48 2 68
0 8 3 3
048 2 80
025
0 55
This shows that the rate of sulfur combination increases with increase in nitrogen content. Recent work by Park and Sebrellt shows that heating rubber increases its plasticity and rate of cure in rubber-sulfur and mercaptobenzothiazole compounds. They heated crude rubber a t 70 pounds steam in open steam and in a closed mold. (Tables I and 11) T a b l e I-Effect
of M e c h a n i c a l T r e a t m e n t a n d H e a t in Plasticity of Smoked Sheets
Relative ~.. plasticity Williams method 4.75 4.3s 4.27 ~~~~
Heat treatment No heat No heat No heat No heat No heat No heat No heat 24 hours open steam 70 pounds 24 hours dry mold a t 70 pounds
Passes through mill set t o 1 mm. h-one 1 2 3 4 5 6 None Sone
4.15 3.96 3.69 3.64 3.69 3.81
in
Recovery thickness Mm. 0.63 0.42 0.33 0.25 0.19 0.16 0.11 0.26 0.19
These figures (Tables I and 11) show that heating increases the amount of acetone extract, and the acid number, the soluble nitrogen, decreases the insoluble nitrogen, and makes the rubber more plastic. The fact that plain sulfur and mercapto cures are speeded up indicates t h a t the nitrogen converted to the soluble form is combined in organic bases or amino acids.
Examination of the rather scanty data on rubber protein indicates that the greater part of t'he nitrogen in rubber is usually combined in the protein. Some of this nitrogen can PRoTEIxs--m'eberz4 in 1900 determined that the insoluble part of rubber was not hydrocarbon, since i t analyzed C30H680i0. be rendered available, in other forms, by heat. I t seems also to he a fact that, whereas some of the protein is rather readily He found that the portion insoluble in CHCll was 3.5 per cent. Spencez5in 1907 first showed t h a t the insoluble material is separated from rubber, a small portion (0.05 to 0.10 per cent) protein-he thought, glycoprotein. In 1813 Tschirch and SchmitzZ6proposed a method of separat- is separated' only 11-ith the greatest difficulty. The protein ing rubber protein by disintegrating rubber in pentachloro- appears to be made up of various compounds. Some further data on the effect of protein hydration xi11 appear under ethane a t 80' C. and separating prQtein by decantation. They found the protein contained 0.345 per cent nitrogen calculated on structure. The specific chemical behavior of each protein the original rubber out of a total of 0 . 3 5 i per cent. In 1914 Spence and KratzZ7made a separation by using tri- compound occurring in rubber should be investigated; also the effect of their decomposition products resukirig from chloroacetic acid. They found the nitrogen content about the heat and from fermentation. The physical be1iavio.r r d l be touched upon elsewhere. t The Goodyear Tire 8r Rubber Company.
'
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
1142
Vol. 18, No. 11
Table 1 1 - C h a w in Nonrubber Materiel by Heat (24 hours in dry press at 70 pounds) Nitrogen in Extract Per cent Rubber Pale crepe Brown crepe 1 Brown crepe 2 Smoked sheets Sprayed latex (a) before heating.
(6)
3.09 3.86 1.60 2.00 1.60 2.72 3.13 4.28 4.97 5.82 ( b ) = after heating.
-
231 47 109 253 381
300 131 163 305 411
CHEMISTRYOF RUBBER RESINS-In 1907 Ditmars3examined the resins extracted from Congo, Madagascar, and Borneo rubbers and found t h a t they contained no free acids. In the same year Edwardoffa' extracted Hevea latex with ether, chloroform, and benzene and tried unsuccessfully to obtain the iodine derivative. Hillen3s in 1913 found lupeol acetate and P-amyrin acetate and a resene in Pontianac resin. He isolated cholesterol acetate from Manihot glaziovii. H e found no phytosterol-like substances in guayule, b u t considered the extract to be ethereal oils. Gutta-percha contained lupeol cinnamate, an oleaginous material, and a little resene. In 1910 Hinrichson and Marcussonas found rubber resins to be optically active, a property which they ascribed t o the unsaponifiable portions. They also found t h a t acetone-soluble oxidation products of rubber are optically active. After examining the resin from twenty-six different species they reported Hevea resins inactive optically. This is now contradicted by Whitby.8' They found 25 per cent of the Hevea resins to be unsaponifiable. This was confirmed by van Rossema8in 1918 and by DekkeraD in the same year. Beadle and Stevens40 in 1912 report that part of Hevea resin is water soluble. Spence and Kratz4*proved t h a t the acetone extract contains nitrogek In 1925 Bruni42 separated aliphatic acids from slab-rubber resins, particularly valeric acid, also valeramid. Dekker43in 1925 studied the fatty acids of Hevea resin. H e found t h a t the water-soluble portion of acetone extract contained formic acid and probably butyric. From saponification of extract he isolated stearic, palmitic, oleic, and linoleic acids. T h e most comprehensive work on rubber resins has been reported by W h i t b ~ . H ~ e~ reports: sterol ester, sterol glucoside, sterol, d-valine, quebrachitol, stearic acid, oleic acid, and linoleic acid. Whitby points out t h a t the per cent unsaponifiable on the rubber
/
1
3
4
5
6
7
8
d
/O
is nearly constant although the total extract varies widely. In acetone extracts, where t h e unsaponifiable portion varied from 22 t o 48 per cent of the total extract, the percentage figured on t h e rubber varied only from 0.71 t o 0.87 per cent. He also mentions that rubber, raw and vulcanized, swells in oleic and linoleic, molten stearic, or the mixed liquid fatty acids isolated from Hevea rubber. Smoked sheet rubber in the mixture took up 4.04 times its weight in 7 days. Since this work is discussed in detail by Dr. Whitby,$ we will not dwell on i t here any more than t o say it has been repeated and corroborated by Sebrell and Carson.? Very little attention has been paid t o the unsaponifiable portions of Hevea extract by other investigators. However, this has been investigated by Carson,t who first showed t h a t t h e $ Page ll6$ of this issue.
0.05 0.05 0.08 0.23 0.13
(b) 0.16 0.11 0.15 0.38 0.20
Nitrogen in Residue Pe.r cent (a)
0.41 0.31 0.47 0.58 0.54
(b) 0.34 0.22 0.31 0.40 0.38
-Plasticity(a)
4.66 3.70 3.67 5.65 5.02
(b) 3.31 1.96
2.53 3.45 2.82
unsaponifiable portion contains material which is responsible for protection of rubber against oxidation. This was further investigated by Bruson,t also of our laboratories, who separated the unsaponifiable portion into three fractions: (1) fragrant smelling, colorless oil, consisting of a mixture of ketones boiling from 60-110" C. at 3 mm.; (2) white crystalline solid, melting point 57-58' C., boiling point 196-200" C., at 3 mm. This has been positively identified as. octadecyl alcohol, by analysis, by mixed melting points, preparation of its acetate (melting point 31' C.) and by oxidation t o stearic acid; (3) transparent, reddish, viscous oil, which is the natural antioxidant of Hevea rubber and possesses the following properties: boiling point 258-260' C. at 1.9 mm.; n y o = 1.5395; very soluble in organic sclvents, insoluble in water; shows no tendency toward crystallization and is optically inactive. It possesses the formula C2rHd20~ (checked by analysis and molecular weight) and is alcoholic in nature. Acetylation a t 100' C. with acetic anhydride shows the presence of one reactive hydroxyl group. On standing in the air it darkens in color. Oxidation with chromic acid yields a mixture of fragrant smelling acids. It occurs in crude rubber to the extent of 0.08 per cent.
These data show that the saponifiable portion of the acetone extract has been thoroughly investigated and its components identified. This portion of the extract is quite variable in amount and composition. The unsaponifiable portion has received less attention but its components have been isolated and the powerful natural antioxidant isolated. Further work on the effect of the resin constituents and the degree of their variation should be carried out. The S t r u c t u r e of Hevea R u b b e r
THE LATEX-Hauser's and Freundlich's46 study of latex showed the rubber particles t o be small, pear-shaped units, sometimes as large as 3p, with a n outer layer of protein, overlying a shell of stiff hydrocarbon and a n inner mass of softer hydrocarbon. Hauser shows t h a t the particles in balata latex have a stiffer shell and t h a t the inside mass is a stiff paste. Ficus elasticus, Castilla, and synthetic rubbers, on the other hand, do not have outer skins. The three latter all show the properties of rubber t o a greater or less qxtent. A particle resembling the latex particle has been redeveloped by Kelly and Rowlandt from a 1 per cent benzene solution of pale crepe by mixing with alcohol and adding water and boiling off volatile solvents. The resulting colloidal solution has same isoelectric point as original latex. This points to a continuous protein film on the particle and not an adsorbed layer. Kelly points out t h a t 0.03 t o 0.04 per cent nitrogen on the rubber is sufficient t o supply a monomolecular film of protein for all the original latex particles. This shows the desirability of removing all the protein before studying the properties of pure rubber hydrocarbon. The isoelectric point of ammonia-preserved latex has been determined by Rowland t o be p H = 3.4. This is also the point of minimum hydration. Thus catophoretic measurements can be used to measure protein hydration. Aging of latex in the absence of bacteria gives diminishing ability t o bind water, according t o Rowland. This greatly diluted Hevea latex was dialyzed t o remove electrolytes and mobility determinations were made at three intervals-immediately after dialysis, a t the end of seven days, and a t the end of ten days. A t the time of determining mobility a small sample of the latex was removed and brought up to p H 9.5 with ammonia. All water used was saturated with toluene t o prevent bacterial action. The results showed t h a t the mobility decreased 25 per cent in seven days and in the succeeding three days decreased 7.7 per cent more, based on original mobility. (Figure 1) Coagulation is apparently due t o the coagulation of the protein. Coagulation does not fuse the latex particles (Sebrell, Psrk, and Martin).4e UNMILLED RvsarER--UnmiUed rubher treated With ammonium hydroxide shows opacity. Barium chloride inhibits the effect.
Table Ill-Rates 1000 c.
Rubber pH = 3 pH = 12
0.08
0.10
1143
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1926
of Dehydration of Two Rubbers Made f r o m Same Latex Coagulated at Two pH Values (Rowland and Tarplint) PER CENT OF Ha0 RBMOVSD AT TEMPERATURE INDICATED 1100 c. 1200 c 130’ C. 140’ C. l%500c. 160’ C. 170’ C. 0.013 0.005 0.000 0.039 0.034 0.044 0 .‘is2 0:003 0.010 0.015
7
180’
... ...
Milled rubber does not show the effect in any reasonable time. This shows the protein phase t o be continuous in unmilled rubber but not in milled. Unmilled rubber absorbs more water and absorbs it faster than milled rubber. Thus it would seem that coagulum is a sponge-like structure of protein filled with hydrocarbon, whereas milled rubber has the hydrocarbon as the external phase. Green” has shown photographs indicating t h a t a latex film (unvulcanized or vulcanized with sulfur chloride) when stretched exhibits a high degree of stretch in the outer skins of the particles. This differs from Hauser’s observations regarding the outer shell of the particles and should be carefully investigated. Klein and Stamberger48 show t h a t solvent swells unmilled rubber to a fixed volume. This is evidence of a two-phase system. Unmilled rubber solution is homogeneous under the ultra microscope, while milled rubber gives a solution with a vast number of particles in Brownian motion, according to Klein and Stamber ger Protein oriented a t the interface, according to Rowland, exerts a marked influence on the physical properties of rubber and is responsible for its strength in t h e unmilled state. The stiffness, toughness, and appearance change with the bound water content. Thus rubbers which are hydrated are soft, pliable, and opaque, while dehydrated rubbers are tough, hard, and transparent. Both can be made from the same latex. R a t e of cure and quality of cured stock are affected by bound water, a t least on alkaline side. (Tables I11 and IV)
C.
0:004
ter’s. The melting point of the “ether soluble” is 115-130° C.; that of the “ether insoluble” is 145-160” C. In our laboratories, Rowland, using the method of‘Pummerer except that he materially increased his times of treatment, has been unable to prepare absolutely protein-free rubber. Rubber obtained in this way analyzes 0.17 per cent nitrogen. Hauser also reports in private communication that he separates rubber purified absolutely free from protein by a refinement of Pummerer’s method into two fractions, a n ether soluble approximately 70 per cent and insoluble approximately 30 per cent. These two fractions cured separately give poor rubber. Together they are approximately equal t o original. After standing they reach a new equilibrium whereby the insoluble becomes partly soluble, and vice versa. The new rubbers are both equal t o the original. This work is unfinished and will be published in full.
.
Table IV-Influence of pH a t Coagulation on Rate of Cure (Sebrell, Carson, and Zimmermant) Best cure Rubber-Sulfur Mercapto D. P. G. Hexa Rubber Min. at 40 lbs. Min. at 20 Ibs. Min. at 40 lbs. Min. at 40 Ibs. pH = 1.0 170 50 50 40 pH = 3.0 100 40 40 40 140 60 40 40 pH = 4.8 80 30 30 SO pH 14.0 9.0 60 20 30 60
z
49 43
92 41
90
39 3B 37
P o ~ n j a khas ~ ~ shown there is a n equilibrium between the solvent taken up by unmilled crude rubber and the external pressure. This indicates t h a t the protein acts as a semipermeable membrane and is a t variance with Feuchter’s conclusions. MILLED RUBBER-Milled rubber gives colloidal particles in benzene solution. It absorbs water very slowly. The work of Kelly and Kurtz, of the writer’s laboratories, on rate of reaction of rubber sols with sulfur chloride shows the amount of milling does not affect the rate. (Figure 2) This is confirmed by figures on sulfur addition in hot vulcanization. They conclude from this that rubber does not depolymerize during milling. Milled rubber disperses in solvent with little swelling. PROTEIN-POOR RuBBER-caspariKO was one of the first to attempt a quantitative fractionation of rubber by petroleum ether into soluble and insoluble portions, approximately 70 per cent soluble and 30 per cent insoluble. The soluble portion swells little and goes readily into solution and the insoluble portion swells greatly and dissolves little in petroleum ether. Both portions are soluble in chloroform and other common rubber solvents. He notes a difference in the behavior of various rubbers. FeuchterKiprepared a “diffusion rubber” by treating unmilled rubber with ether and other solvents. Swelling of the rubber first takes place, then gradual thickening of surrounding solvent, The residue maintains its sharp boundaries and he calls it the “gel skeleton.” It is tough, leathery, and inelastic. The diffusion rubber is highly elastic and shows the properties of an improved rubber. Feuchter says it contains no protein skins and, indeed, i t is difficult t o see how the protein skins could get loose from the protein structure without the disintegration of the latter. This is highly important work in t h a t it carries the implication t h a t the elastic properties of rubber do not reside in the latex skins. The writer’s enthusiasm for this work is dimmed by the somewhat incoherent theory which accompanies it, but confirmation of the experimental work is not lacking, PummererK2prepared a purified rubber by heating latex in caustic t o hydrolyze the protein, subsequently coagulating and extracting with acetone. He claims that this rubber is free from protein and ash and analyzes quite exactly C&. H e separates this purified rubber into “ether-soluble’’ and “etherinsoluble” portions hy a diffusion process very similar t o Feuch-
0
/
P
3
4
3-
6
7
CALENDERANn STRETCHE~FEcT-DeVisser~~shows that calendering and quick cooling, while shrinkage is prevented, produces a rubber stiffer in direction of calendering than across. It shows double diffraction, dichroism, increased specific gravity, hardening on storage, and x-ray pattern. It should be noted t h a t high-speed, high-pressure conditions produce the effect, and slow, low-pressure conditions remove it a t a temperature lower than temperature of formation. HockK4shows t h a t rubber stretched and frozen shatters t o give pseudo fibers. Rubber stretched and cooled to 10-15’ C. gets hard, inelastic, and opaque, and tears readily to give threads which curl up spirally. Vogtt gives specific gravity increase 0.922 t o 0.939 a t 9.2’ C., when stabilized a t 550 per cent stretch. Kat266 shows t h a t stretched rubber gives an x-ray pattern and he estimates a unit cell of the approximate dimensions 8.x 6!/2 X 61/2Rngstrijm units. Ott6‘ finds t h a t the largest dimension of the unit cell is 6.37 A. ClarkK7agrees with this and computes the number of isoprene units as six. Pummerer and HockK8give the unit cell volume for rubber crystals as 118.7 A3. and conclude t h a t it contains but one isoprene molecule. Hauser and MarkKgplace the size of the unit cell at 8 x 8.6 x 7.68 A., or double this when the rubber is strongly stretched.
The discrepancies here noted are worth brief comment. Table V shows the values assigned by different investigators to the cell volume and to the volume of the isoprene molecule. Pummerer and Koch Ott Clark and Lsnyon Ratz Hauser and Mark
Table V Unit c:ell 118.7 dis. 259 259 338 (apNprox.) 529
Isoprene molecule 120 A? 43 43 120 122
It should be pointed out that the value assigned by Ott, Clark, and Lanyon to the molecular volume of isoprene, is
1144
INDUSTRIAL AND ENGINEERING CHEMISTRY
such that we must conclude that rubber is made u p of units the specific gravity of which is about three times that of rubber in the mass. This would require the postulation of another phase very much lighter in gravity-an unlikely hypothesis in view of no separation of two substances of unlike gravities ever having been made from rubber. If we assume, as most investigators have, that the isoprene molecule in rubber has the same specific gravity as rubber and weighs 68.0 times as much as the hydrogen atom, we find that Pummerer and Koch require one isoprene per cell, Clark and Lanyon require two, Katz requires three, and Hauser and Mark require four. Doubtless there are others to be heard from!
Rubber when stretched gives up heat, the heat passing through a maximum and then falling off (Figure 3). The effect is thermodynamically reversible. This Joule effect is one of the most interesting of the rubber properties and needs further quantitative study. A negative effect a t low elongations and low temperatures is noted by Joule and others. Poisson's ratio for rubber is 0.5. This is quite as characteristic as any of the rubber properties. Cured synthetic rubber gives the same shaped stress-strain curve as cured Hevea rubber, but it is shorter and stiffer (Figure 4). Both stress-strain curves are reverse curves and the slope at, low elongations is as high as a t the end of the curves. TEMPERATURE TRANsITIoNS-fi~pha Transition. Rubber frozen in liquid air is transparent and brittle like glass.60 It reverts quickly below 0" C. Beta Transition. Rubber stretchedo' and cooled to 10-15' C is stabilized-i. e , becomes hard, inelastic, opaque, and anisotropic, and of higher specific gravity. It reverts quickly a t 18' c. Gamma Transition. On long storage a t 0-10' C. rubber becomes opaque and hard. It reverts slowly a t 25' C., and quickly at 30' C. It is interesting to note t h a t using the Debye x-ray method natural rubber gives amorphous rings, but when stretched it gives parabolic bands. Balata, on the other hand, gives concentric rings of uniform intensity, which are characteristic of random crystal arrangement. It is possible t h a t naturally frozen rubber-i. e., rubber in the gamma stage-will give an x-ray diagram like balata. Delta Transition. (1) Rubber will not give telescopic flow in tube machine below 50-60' C.62 (2) Calender effect disappears63 a t 45-47' C. (3) Milled rubber64 will show a tendency to reaggregate a t 50-60" C. of stretched rubber disappears a t 60" C. temperatures m a y be the same time factor. Alpha has stress-stnin curve
VOl. 18, No. 11
like balata;6Egamma has specific gravity change like balata (see Figure 5) ; gamma has stress-strain curve like loaded rubber. ISOMERIC FORMS-staudinger obtained isomeric forms by heating. Fisher, Gray, and McColm also did this. Kirchhof obtained isomeric forms by heating with sulfuric acid. Bruson obtained isomeric forms by catalysts Balata, dimethylbutadiene-polymer, and rubber give the same chemical reactions.
Discussion
A study of the foregoing facts leads to a number of questions regarding their implication and concerning the nature of facts related but a t present unknown. For example, although we know that rubber is a polymer of isoprene we do not know how the isoprene units are connected and whether they form a straight chain or a ring. Nor are we positive that there is more than one polymer present in rubber, although the evidence indicates two. Again, the relationship between Hevea rubber, balata, and polymerized isoprene must be significant, but just what does it signify? Is the difference merely one of the size of the polymers? There is some evidence of this. For example, balata is stable a t room temperature and when stretched has a high set. Hevea rubber when stretched is stabilized a t 10" to 15" C., although vulcanization greatly lowers this temperature. Synthetic rubber is stabilized a t about - 15" C. Does this pmgressive decrease of the temperature of stabilizatiof indicate a corresponding decrease in the size of the polymer? If we turn to a contemplation of the profound changes which are brought about by internal saturation, we are led to wonder if this is related to the less profound change due to vulcanization. Or if we consider the protein in rubber we may ask whether it has an effect on the structural behavior or merely on vulcanization and aging. It is apparent that the last traces of protein are difficult to remove, but we are uncertain whether this trace is significant or not. Thus we know that an evaporated latex film is elastic and vulcanizes to give a n elastic film; likewise a protein-poor film exhibits elastic properties. This may mean that the protein is un-
70
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important or that only a small amount is needed to produce the result. Studies of absolutely protein-free rubber would settle this question. Or if we turn to the latex particles we are faced with the question as to whether these particles persist in milled rubbei. and to what degree rubber properties are dependent on such persistence. ?'he phase separation of Hauser and others seems to east doubt on such persistence, but the work of Kelly and Rowland in redispersing rubber is contrary evidence.
1145
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1926
Are the outer skins of the latex particles capable of high stretch, as indicated by Green’s work, or not? Consider the acetone-extractable material. The fact that the resin acids swell rubber may account for some of its behavior when stretched and examined under the x-ray, to say nothing of milling and vulcanization phenomena. Moreover, the action of the natural and of other analogous antioxidants raises new questions. Excessively vulcanized rubber oxidizes readily and 011 aging its tear resistance and tensile product fall off rapidly a t the same rate. The presence of an antioxidant retards oxidation and decrease in tensile product, but not the decrease in tear resistance. What structural behavior would account for this?
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We are further curious to know whether the et her-soluble and insoluble portions of rubber obtained by IIauser and others exhibit Joule effect and give x-ray patterns on stretching if examined while fllesh. Furthermore, can synthetic rubber be fractionated by ether and, if so, will an equilibrium be established? Then there arises a question in connection with that most peculiar of all rubber phenomena, the Joule effect-is the crystal formation indicated by the x-ray a cause for the shape of the Joule heat curve? This question follows from our knowledge of the tendency of crystalline structures to cool upon stretching. Explanation of Joule effect by heat of compression or crystallization alone fails to explain maximum. Finally, we murt not forget that the most remarkable thing about rubber is its high stretch capacity and its almost perfect recovery. Accompanying this is a stress-strain curve of S shape, which is remarkable, first, because it is as stiff a t very low elongations as it is a t much higher dongations, and second, because increasing stretch requires progressirely increasing load. KO theory is worth a passing thou@ which does not explain these things. I n conclusion, it may he stated that no attempt will be made at this time to advance a n hypothesis for rubber structure. If this brief journey through a maze of seemingly unrelated and sometimes conflicting facts has left any definite impression it is that more data (much of it easily obtainable) are needed made. That such data before reliable generalization c will be gathered and that a satisfactory theory will result therefrom, we have not the slightest doubt. Acknowledgment
The collection of data for this paper rapresents the composite efforts of the mseareb ski% e Goodyear Tiie 8: Rubber Company. , 1
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Bibliography 1-Proc. Roy. Soc. ( L o n d o n ) , 10, 516 (1860). 2-Comfit. rend., 89, 361 (1879). 3-Ann., 225, 311 ( 1 8 8 4 ) ; 227, 238, 292 (1885). 4-Chem. News, 65, 265 (1892). S-Ber., 36, 3260 (1902). 6-J. Chem. Soc. ( L o n d o n ) , 53, 679 (1888). 7--Ber., 37, 2708 (1904). 8--lbid., 38, 1975 (1905). 9-Ibid., 38, 1195 (1905). 1 0 - 4 . Chem. Scc. ( L o n d o n ) , 97, 1085T (1910). 11-Comfit. vend., 154, 1159 (1912). 12-Wiss. Vertfenllich. S i e m e n s - K o n z w n , 1.2, Heft 87. 13-Ber., 65, 3458 (1922). 14-Helzelica Chim. A c t a , 5, 785 ( 1 9 2 2 ) ;C. A . 19, 2974 (1925). 15-Ifclvelica Chim Acta, 7, 842 (1924). In-Ibid., 9 , 549 (1926). 17-Ind. Eng. Chem., 18, 414 (1926). 18-Helvetica Chim. Acta, 9, 529 (1926). 39, 2143 (1926). 19-Gurnmi-Ztp., 20-Kautschuk, January 1 , 1926, p . 1. 21-Haluelica Chim. A c t a , 9, 549 ( l b 2 6 ) . 22-3. A m . Chem. Soc., 48, 1309 (1926). 39, 2143 (1926). 23-Gummi-Ztg., 24-3. Soc. Chem. ( L o n d o n ) , 2, 215 (1900). 25-@uarl. J . Inst. C o m m . Res. Tropics, Liverpool, 1907. 26-Gummi-Ztg., 26, 2097 (1913). 14, 262 (1914). 27--Kolloid-Z., 28-Meded. Delft, 1916, 483. 29-Rubber I n d . , 1914, 144. 30-Malayan A g r . J . , 13, 154 (1925). 31-Biochem. J . , 5, 1106 (1911). 32--Kolloid-Z., 11, 6 1 (1912). 33--Gummi-Ztg., 21, 670 (1907). 34-Ibid., 21, 635 (1907). 35-.4rch. Phavm., 251, 94 (1913). angew. Chem., 23, 49 (1910). 36-Z. 37-J. Chem. Soc. ( L o n d o n ) , 129, 1448 (1926). 38-Kolloidchem. Beihefle, 10, 1 (1918). 10, 54 (1918). 39-Ibid., 40-Intern. Cong. Afipl. Chem., 24, 581 (1912). 41-Kolloid-Z., 14, 268 (1914). 42-Giorn. chim. ind. applicata, 7, 447 (1925). 43-India Rubber J . , 70, 815 (1925). 44-J. Chem. Soc. ( L o n d o n ) , 129, 1448 (1926). 45-India Rubber J . , 69, 663 (1925). Eng. Chem., 17, 1173 (1925). 46-Ind. 47-Ibid., 17, 802 (1925). 48-Kolloid-Z., 36, 362 (1924). 49-Kolloidchem. Beihefte, 3, 417 (1912). 50-3. Soc. Chem. I n d . , 32, 1041 (1913). 51-Kolloidchem, Beihefte, 20, 434 (1925). 52-Kaulschuk, 1926, 85. 53-J. H. D e Bussy, Dissertation Venlo, Amsterdam, Holland. 54-Gummi-Ztg., 39, 1740 (1925). 55-Chem -Ztg., 49, 353 (1925) also Kolloid-Z., 37, 19 (1925). j 6 - ~ ~ f u r v i s s e n s c h a f l e n ,15, 320 (1926). 57-Colloid Symposium, June, 1926. 438, 294 (1924). S-Ann., 59-Kolloidchem. Beiheffe, 22, 77 (1926). 60-Hock, Gummi-Ztg., 39,1740 (1925). nl-Hock, Kolloid-Z., 36, 40 (1924). 62--Marzetti, Giorn. chim. ind. applicata, 6, 567 (1924). Visser, Dissertation Venlo. 63-De 64--Griffiths, Trans. Inst. Rubber I n d . , 1, 308 (1926). 65-Hauser a n d Mark, Kolloidchcm. Beihefte, 22, 63 (1926). 66-Park, I n d . Eng. Chem., 17, 152 (1925). ~
42. R. Downs to Receive Grasselli Medal Charles Raymond Downs, of Weiss & Downs, consulting chemists, New York City, has been selected as the 1926 recipient of the Grasselli Medal, which will be presented to him on November 5 a t a joint session of the American Section of the Society of Chemical Industry and the New York Sections of the AMERICAN CHGMICAL SOCIETY, the American Electrochemical Society, and the SociCt6 de Chimie Industrielle in Rumford Hall, New York City. Treat B. Johnson will deliver an address on “The hiedalist and the Award.” Dr. Downs’s paper will have to do with “Some Speculations in Catalytic Oxidation Reactions ” Gerald Wendt will discuss the “Phenomena Related to Catalysis.” During the war Dr. Downs did a great d e d Of work on catalytic oxidation in the vapor phase and patented a method of oxidizing benzene to give malic acid.
